Oxygen Ion Butterfly Distributions Observed in a Magnetotail Dipolarizing Flux Bundle

摘要

Cluster observed two intermittent oxygen ion (O~+) flux enhancements with energy dispersions in a dipolarizing flux bundle, which is known as a region of enhanced northward magnetic field (B_z) embedded in the earthward high‐speed flow. The flux enhancements of O~+ show clear pitch angle dependences, which are termed as butterfly distributions. Two corresponding flux enhancements of field‐aligned protons (H~+) are also shown in its spectrum, but they are weaker and emerge later (~10 s) than those of O~+. Simulation shows that both enhanced ion species are the counterstreaming populations. They originated from the lobe region and were driven into the center plasma sheet by the dawn‐dusk electric field (Ey). Backward tracing test‐particle simulations reproduce the butterflyO~+ and the counterstreaming H ~+ distribution. The differences between O~+ and H~+ are because of their different gyroradii. The lobe O~+ can arrive at the magnetic equatorial plane in less than one gyromotion due to its large gyroradius, and O ~+ with a larger field‐aligned velocity can arrive at the equatorial plane earlier, leading to the energy and pitch angle dependence. While H~+ with similar energy can drift into dipolarizing flux bundle through electric field drift (E × B motion) and arrive at the equatorial plane through adiabatic motion, which consequently forms the field‐aligned flux enhancements in dipolarizing flux bundle, that is, the B——z‐dominant region. The simulation further confirms that intermittent increases of Ey component can produce the two intermittent flux enhancements, as indicated in the in situ observation.